The function of importin β1 is conserved in eukaryotes but the substrates may vary in organisms

Abstract

Importin β1 is the nuclear-cytoplasmic transport receptor in eukaryotic cells. Its main function is to transport NLS (nuclear localization signal)-containing proteins from cytoplasm to nucleus. Our recent study found that AtKPNB1, a homolog of the human KPNB1, is an essential component of the classical nuclear import of the NLS-containing proteins in Arabidopsis and modulates plant development and ABA-mediated stress response. Human KPNB1 can also directly transport the nuclear proteins, such as ribosomal protein RPS7e, without the intervention of importin α proteins. However, we found that AtKPNB1 does not directly recognize and import the human RPS7e homologous proteins AtRPS7A, AtRPS7B and AtRPS7C into the nucleus like human KPNB1. These findings suggest that the importin β1 protein has the conserved function in translocating nuclear proteins to the nucleus, but their specific cargos may vary in different organisms.

Keywords: Arabidopsis thaliana; NLS-containing protein; importin α; importin β1; nuclear-cytoplasmic transport.

Figure 1. Phylogram of AtRPS7A, AtRPS7B, AtRPS7C and closely related proteins. An unrooted phylogenetic analysis was conducted using MEGA version 5.2. Bootstrap test of phylogeny was performed using the neighbor-joining method from 1,000 replications for each branch. The accession numbers are: AtRPS7A, AT1G48830; AtRPS7B, AT3G02560; AtRPS7C, AT5G16130; Homo sapiens, P62081; Caenorhabditis elegans, Q23312; Drosophila melanogaster, Q8IMI7; Oryza sativa, A2XFL8; Populus trichocarpa, A9PEW2; Saccharomyces cerevisiae, P26786; Triticum urartu, EMS56913; Torulaspora delbrueckii, XP_003682163; Mus musculus, AAB97861.1.

Figure 2. No interaction between the ribosomal proteins and AtKPNB1. (A) The interaction assay between AtKPNB1 and the ribosomal proteins using yeast two hybrid system. (B) The subcellular localizations of AtRPS7 in protoplasts of wild type (WT) and the mutant atkpnb1.